1. Field of the Invention
This invention relates to paper-faced gypsum wallboards having a special core formulation with ingredients intended to aid in maintaining the board integrity when subjected to fire.
2. Description of the Prior Art
U.S. Pat. No. 1,778,008 discloses using pieces of mica or exfoliated zonolite in gypsum board, to make the board light, strong and inexpensive, and states that if the particles are all larger than 16 mesh, the mica will not detract from the strength of the finished product. Particles of up to about one inch in size are recommended, so that they can interlock with one another.
U.S. Pat. No. 2,198,885 discloses the use of granular highly porous calcium silicate, and describes a prior use of expanded flakes of mica, which, while comparatively light in weight and fire-proof, did not possess good internal strength and did not bond to gypsum.
U.S. Pat. Nos. 2,526,066; 2,744,022; 3,376,147; 3,454,456 and 3,616,173 all relate to the use of unexpanded vermiculite, which is a form of mica, in a gypsum wallboard core since it is capable of expanding at high temperature and thus counteracting the tendency of the gypsum in the core to shrink at high temperature, as in a fire.